This invention relates generally to a work piece carrier head for a polishing apparatus, and more specifically to a carrier head having a circumferential wear ring mounted to have a near zero overturning moment.
The manufacture of many types of work pieces requires the substantial planarization of at least one surface of the work piece. Examples of such work pieces that require a planar surface include semiconductor wafers, optical blanks, memory disks, and the like. Without loss of generality, but for ease of description and understanding, the following description of the invention will focus on applications to only one specific type of work piece, namely a semiconductor wafer. The invention, however, is not to be interpreted as being applicable only to semiconductor wafers. Those of skill in the art instead will recognize that the invention can be applied to any generally disk shaped work pieces.
One commonly used technique for planarizing the surface of a work piece is the chemical mechanical planarization (CMP) process. In the CMP process a work piece, held by a work piece carrier head, is pressed against a moving polishing pad in the presence of a polishing slurry. The mechanical abrasion of the surface combined with the chemical interaction of the slurry with the material on the work piece surface ideally produces a planar surface.
The construction of the carrier head and the relative motion between the polishing pad and the carrier head have been extensively engineered in an attempt to achieve a uniform removal of material across the surface of the work piece and hence to achieve the desired planar surface. For example, the carrier head generally includes a flexible membrane that contacts the back or unpolished surface of the work piece and accommodates variations in that surface. One or more pressure chambers may be provided behind the membrane so that different pressures can be applied to various locations on the back surface of the work piece to cause uniform polishing across the front surface of the work piece. The carrier head also generally includes a wear ring (sometimes referred to as a xe2x80x9cretaining ringxe2x80x9d or xe2x80x9cedge ringxe2x80x9d but hereinafter referred to without limitation as a xe2x80x9cwear ringxe2x80x9d) that surrounds the membrane and the work piece and that pre-stresses or pre-compresses the polishing pad to protect the leading edge of the work piece. The height of the wear ring generally, but not always, can be adjusted. The polishing pad may move in a linear motion, a rotational motion, or an orbital motion, depending on the type of CMP apparatus. Additionally, the carrier head, and hence the work piece, may also be in rotational motion. The relative motion between the work piece and the polishing pad is designed to attempt to provide equal polishing to all areas of the polished surface.
Despite all the efforts to achieve uniform polishing across a work piece surface, however, a uniform removal rate has not been obtained. Instead, a xe2x80x9cslow band,xe2x80x9d exists around the edge of the work piece. For example, examination of a semiconductor wafer that has undergone a CMP process exhibits a band around its periphery, spaced inwardly from the edge of the wafer, that has experienced a slower material removal rate than has the remainder of the wafer. The slow band exists regardless of whether the wafer is 200 mm or 300 mm in diameter and regardless of membrane pressures, slurry composition, polishing speed, relative motion, or other CMP conditions. The existence of a slow band reduces the yield of the semiconductor wafer because the slow band causes a non-planar surface, and subsequent processing steps require a substantially planar surface. Lower yield, of course, is undesirable. Accordingly, a need existed for a carrier head for use in a CMP or other polishing process that would overcome the problems of the prior art carrier heads and would produce a uniform planar work piece surface without evidence of a slow band or other surface anomaly.